Diesel is a common fuel product that is widely available worldwide, and is suitable as a fuel for variety of vehicles, such as trucks or ships, among others. Jet fuel, on the other hand, especially that of a quality making it suitable for aircraft, is not as widely available. There exists, therefore, a substantial demand for methods which would allow conversion of diesel fuel into jet fuel, for example, using catalytic hydrocracking.
Hydrocracking is a process combining catalytic cracking and hydrogenation of heavy feedstock, which is cracked in the presence of hydrogen to produce lighter products, e.g., isobutane for alkylation feedstock, and various other hydrogenated products. These products boil over a lower range of temperatures than the feed stock. The process is usually carried out under quite harsh conditions of high pressure and high temperature, and uses a catalyst. Traditional catalysts that are often employed include dual function catalysts that are useful for both acidic cracking (provided by a catalyst component that is an amorphous silica-alumina support or a crystalline zeolite material) and metallic hydrogenation (provided by a metal sulfide component incorporating such metals as nickel, tungsten, cobalt or molybdenum finely dispersed on the support material).
As the conversion of the hydrocracking processings increased, lighter (lower-boiling) products are formed. At low conversion the primary products are in the gas oil and diesel range. As conversion is increased more material is formed in the kerosene range, then in the naphtha range and finally in the range of butanes, propane and light hydrocarbons.
Because the lighter, lower-boiling compounds that are formed at higher conversion in a hydrocracking process contain more hydrogen per carbon atom than the feedstock, the hydrogen consumption of the process increases with the extent of conversion. Hydrogen for hydrocracking processes in oil refineries typically either comes from hydrogen that is produced as a byproduct of the catalytic reforming of heavy naphtha, or else hydrogen that is produced by steam reforming of either natural gas, refinery fuel gas or naphtha. The hydrogen that is produced in these processes is usually purified by pressure swing adsorption to achieve a hydrogen concentration greater than 99.9%, before being compressed to the pressure at which the hydrocracking process operates.
The hydrocracking process can be briefly outlined as follows. First, preliminarily heated feedstock is mixed with fresh and/or recycled hydrogen and sent to a reactor, where sulfur- and nitrogen-containing compounds are removed after being converted into hydrogen sulfide and ammonia. Limited hydrocracking also occurs at this stage. Next, the hydrocarbon is cooled, liquefied and run through a hydrocarbon separator. The hydrogen is recycled to the feedstock, and the liquid is run through a fractionator. The fractionator bottoms are again mixed with a hydrogen stream and the process is repeated.
Accordingly, catalytic hydrocracking is useful for converting the high molecular weight components in heavy petroleum distillates and involves the processes of hydrogenation and carbon-carbon bond cleavage. At the same time, at least a majority of oxygen, sulfur, and/or nitrogen-containing compounds, if any are present, are removed, and olefins are typically saturated to yield paraffins.
As described above, it is desirable to be able to convert diesel fuel into jet fuel. For example, during military or humanitarian operations there may be a greater local need for jet fuel than the available supply of jet fuel, whereas there may be a surplus of local diesel fuel available. In this situation, it may be desirable to convert diesel fuel into jet fuel using a modular plant that can be quickly assembled and is self-contained.